Passive optical network system and optical line terminal

ABSTRACT

In order to be able to moderate the inclination of the PON burst reception characteristics and to improve the FEC effect, a first offset is used in a ranging window field, and a second offset, which is lower than the first offset value, is used in a burst data field other than the ranging window field.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Continuation of U.S. application Ser. No.12/022,172, filed Jan. 30, 2008, which claims priority from JapanesePatent Application No. 2007-203298, filed Aug. 3, 2007, the contents ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a Passive Optical Network (PON) systemin which plural subscriber connection apparatuses share an opticaltransmission line, and an optical line terminal (OLT). Moreparticularly, the present invention relates to a PON system and an OLT,in which the inclination of burst reception characteristics is moderate.

A PON system includes an OLT and plural Optical Network Units (ONUs).Each ONU converts an electrical signal from a terminal (such as PC)connected to the ONU into an optical signal. The individual ONUstransmit optical signals to the OLT through their subscriber opticalfibers and an optical splitter. At this time, the optical signals aretime-division multiplexed on the trunk fiber to the OLT. The OLTprovides communication between a terminal of an ONU, and a terminal of adifferent ONU or a terminal of a different network (NW).

As specified in Sections 8 and 9 of ITU-T Recommendation G984.1, eachONU is located in one of the following three ranges: 0 to 20 km, 20 kmto 40 km, and 40 km to 60 km in length of the optical fiber. However,the transmission distances of the nearest ONU and the farthest ONUdiffer as much as 20 km, and the transmission delays differtherebetween, so that the optical signals output from the ONUs maycollide and interfere with each other. For this reason, the delays ofthe output signals from the ONUs are adjusted as if all the ONUs arelocated at an equal distance (such as 20 km) by the ranging technologyspecified in Section 10 of ITU-T Recommendation G.984.3. As a result,the optical signals from the ONUs do not interfere on the trunk fiber.Incidentally, the PON system can adjust the interference, but cannotadjust the attenuation due to the difference of the length of theoptical fiber.

Further, as specified in Section 8.3.3 of ITU-T Recommendation G.984.2,a guard time, a preamble, and a delimiter are added to the head of thesignal from the ONU. The guard time has 12 bytes and serves as aprotection against the interference. The preamble is used fordetermination of the identification threshold of a receiver, as well asfor clock extraction. The demiliter is used for identifying the boundaryof the received signal.

In Section 8.2 of ITU-T Recommendation G.984.3, signals transmitted fromplural ONUs to an OLT are referred to as upstream signals. The upstreamsignal includes a preamble, a delimiter, and a payload signal. Further,as shown in FIG. 8-2 of Section 8 of the recommendation, a guard time isprovided immediately before each upstream signal in order to avoidcollision with the last bust signal.

On the other hand, according to Section 8.1 of the recommendation,signals transmitted from the OLT to the plural ONUs are referred to asdownstream signals. The downstream signal includes a framesynchronization pattern, a PLOAM field, a US Bandwidth MAP field, and aframe payload. As shown in Section 8.1.3.6 of the recommendation, theOLT specifies the timing of the upstream transmission permission foreach ONU by use of the field called US Bandwidth MAP. The US BandwidthMAP field includes a start value for specifying the start of thetransmission permission, and an end value for specifying the end of thetransmission permission, respectively on a per-byte basis. The valuesare also referred to as grant values, meaning that the transmission ispermitted. The difference between the end value and the next start valueis an upstream no-signal field corresponding to the guard time.Incidentally, it is possible to allocate plural bandwidth allocationunits called T-CONTs to each ONU. The upstream transmission permissiontiming is specified for each T-CONT.

Ranging is performed in such a way that the OLT requests the ONU totransmit a distance measurement signal. The ONU returns a distancemeasurement frame to the OLT. Upon receiving the signal, the OLTmeasures the time period from the transmission request of the distancemeasurement signal to the reception of the distance measurement signal,namely, the OLT measures the round-trip delay time to discover how farthe ONU is from the OLT. Next, the OLT transmits a message to each ONUto delay its transmission by a time called equalization delay so thatall the ONUs appear to be located at an equal distance from the OLT. Forexample, the OLT specifies, for each ONU, an equalization delay that isequal to “(20 km round-trip delay time)-(measured round-trip delaytime)” so that all the ONUs have the 20 km round-trip delay time. TheONU has a circuit for transmitting data with a delay fixed to thespecified equalization delay. The above specification ensures that allthe ONUs have the round-trip delay time of 20 km for the upstream datatransmission.

In JP-A No. 2007-036920, there is a detailed description on the rangingin the PON system described above.

In such a PON burst receiving circuit, an offset is added to a thresholdfor determining “1” or “0”, in order to guard against white noise(hereinafter referred to as noise) occurring in a no signal timeslot,called a ranging window that is used for the ranging. Because the offsetis added to the threshold that would have been in the middle of the peakand bottom values of the received signal, the threshold approaches thepeak value by the amount of the offset. Hence, the probability ofmisidentifying “1” as “0” is higher than when the offset is not used,and the inclination of the burst reception characteristics is steep.Meanwhile, ITU-T Recommendation G.984.3 specifies Forward ErrorCorrection (FEC) technology that can correct an error rate of 1 e-4 toan equivalent of 1 e-12, using a Reed-Solomon code. Larger coding gaincan be obtained as the inclination of the PON burst receptioncharacteristics is moderate.

SUMMARY OF THE INVENTION

The present invention moderates the inclination of the burst receptioncharacteristics in order to obtain larger coding gain in PON bustreception. Further, the present invention provides a PON system and anoptical line terminal that have excellent burst receptioncharacteristics.

The above can be achieved by switching between offsets, one for rangingtransmission in a ranging window and the other for burst data of atimeslot other than the ranging window. A first value is used for theoffset for ranging transmission, and a second value, which is lower thanthe first value, is used for the offset for burst data other than theranging transmission.

Further, the above can be achieved by a PON system including: an opticalline terminal; a trunk fiber connected to the optical line terminal; anoptical splitter connected to the trunk fiber; and plural opticalnetwork units connected to the optical splitter through pluralsubscriber fibers. The optical line terminal is configured to be able toselect an offset to be used to guard against noise occurring in anupstream no-signal field.

Further, the above can be achieved by an optical line terminalincluding: an OE converter; an identification unit for identifying theoutput of the OE converter; and a controller for controlling theidentification unit. The controller controls in the period of receivinga ranging transmission so that the identification unit selects a firstoffset, and controls in the period other than a period of receiving theranging transmission so that the identification unit selects a secondoffset. The identification unit calculates a threshold foridentification based on the first offset or the second offset.

Still further, the above can be achieved by an optical line terminalincluding: an OE converter; a first identification unit for identifyingthe output of the OE converter by use of a first offset; a secondidentification unit for identifying the output of the OE converter byuse of a second offset; a selector for selecting one of the firstidentification unit and the second identification unit; and a controllerfor controlling the selector. The controller selects the firstidentification unit in the period of receiving a ranging transmission,while selecting the second identification unit in a period other thanthe period of receiving the ranging transmission.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described inconjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram of an optical access network;

FIG. 2 is a block diagram of an OLT;

FIG. 3 is a block diagram of an ONU;

FIG. 4 is a block diagram of a PON transceiver block of the OLT;

FIG. 5 is a block diagram of a PON transceiver block of the ONU;

FIGS. 6A and 6B are block diagrams of a PON reception unit and a PONtransmission unit in the OLT;

FIGS. 7A and 7B are block diagrams of a PON reception unit and a PONtransmission unit in the ONU;

FIG. 8 is a functional block diagram of the OLT;

FIG. 9 is a hardware block diagram of O/E and ATC that constitute an OLToptical signal reception part;

FIG. 10 is a block diagram of the ATC and peripheral circuits (part 1);

FIG. 11 is a block diagram of the ATC and peripheral circuits (part 2);

FIG. 12 is a diagram illustrating the reception of upstream burstsignals and ranging transmissions with a constant offset;

FIG. 13 is a diagram illustrating the reception of upstream signals andranging transmissions with different offsets;

FIGS. 14A and 14B are diagrams each illustrating the peak value, bottomvalue, threshold, and offset of a received bust signal with a highoffset;

FIGS. 15A and 15B are diagrams each illustrating the peak value, bottomvalue, threshold, and offset of a received bust signal with a lowoffset; and

FIG. 16 is a diagram illustrating the relationship between the errorrate and the received optical power, with the amount of offset given asthe parameter.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, a preferred embodiment will be described by examples withreference to the drawings. Like or corresponding parts are denoted bythe same reference numerals and the description will not be repeated.

FIG. 1 is a block diagram of an optical access network. An opticalaccess network 10 includes an OLT 1, ONUs 2, a splitter 3, a trunk fiber8 between the OLT 1 and the splitter 3, and subscriber fibers 9 betweenthe splitter 3 and the ONUs 2. Each ONU 2 is connected to an IP system 4and a TDM system 5. The OLT 1 is connected to an IP network 6 and a TDMnetwork 7.

TDM signals from the TDM systems 5 are accommodated in the TDM network 7through the optical access network 10. Signals from the IP systems 4 areaccommodated in the IP network 7 through the optical access network 10.These signals are referred to as upstream signals.

On the other hand, a TDM signal from the TDM network 7 is accommodatedin the TDM systems 5 through the optical access network 10. A signalfrom the IP network 6 is accommodated in the IP systems 4 through theoptical access network 10. These signals are referred to as downstreamsignals.

Incidentally, in the block diagrams in the following FIGS. 2 to 11described below, the signal flow directions (upstream and downstreamdirections) are the same as in FIG. 1.

FIG. 2 is a block diagram of the OLT. An upstream optical signal fromthe trunk fiber 8 is converted to an electrical signal by aphotoelectric conversion module 71, and is subjected to GEM terminationin an OLT PON transceiver block 72. The converted electrical signal isfurther converted to an Ethernet frame and a TDM signal. The Ethernetframe and the TDM signal are transmitted to the IP network 6 and the TDMnetwork 7 through an Ethernet PHY 73 and a TDM PHY 74, respectively.

Downstream signals from the IP network 6 and the TDM network 7 arereceived by the Ethernet PHY 73 and the TEM PHY 74, respectively, andthen transmitted to the OLT PON transceiver block 72. The OLT PONtransceiver block 72 performs a GEM frame assembly, and transmits to thetrunk fiber 8 through the photoelectric conversion module 71. Here, MPU75 is a microcomputer for controlling the OLT 1, RAM 76 is a randomaccess memory, and a control system interface 77 is an interface forsetting the OLT 1 from the outside.

FIG. 3 is a block diagram of the ONU. The downstream signal from thesubscriber fiber 9 is converted to an electrical signal by aphotoelectric conversion module 81, and is subjected to GEM terminationin an ONU PON transceiver block 82. The ONU PON transceiver block 82converts the converted electrical signal into an Ethernet frame and aTDM signal. The Ethernet frame and the TDM signal are transmitted to theIP system 4 and the TDM system 5 through an Ethernet PHY 83 and a TDMPHY 84, respectively.

The upstream signals from the IP system 4 and the TDM system 5 arereceived in the Ethernet PHY 83 and the TDM PHY 84, respectively, andthen transmitted to the ONU PON transceiver block 82. The ONU PONtransceiver block 82 performs GEM frame assembly, and transmits to thesubscriber fiber 9 through the photoelectric conversion module 81. Here,MPU 85 is a microcomputer for controlling the ONU 2, RAM 86 is a randomaccess memory, and a control system interface 87 is an interface forsetting the ONU 2 from the outside.

FIG. 4 is a block diagram of the PON transceiver block of the OLT. Theupstream PON frame signal from the photoelectric conversion module 71 issubjected to such processes as synchronization and GEM extraction by thePON reception unit 90. The extracted payload is transmitted to areception GEM assembly 91. The reception GEM assembly 91 assembles theGEM that is divided into plural short frames for transmission. Then, theassembled GEM is stored in a reception GEM buffer 92, and is transmittedto either an OLT upstream Ethernet GEM terminator 94 or to an OLTupstream TDM GEM terminator 97, according to the table information of anOLT reception table 93.

The OLT upstream Ethernet GEM terminator 94 extracts an Ethernet framefrom the GEM frame, and transmits the extracted Ethernet frame to theEthernet PHY 73 through an OLT upstream Ethernet interface 95. The OLTupstream TDM GEM terminator 96 extracts a TDM signal from the GEM frame,and transmits the extracted TDM signal to the TDM PHY 74 through an OLTupstream TDM interface 97 at a desired timing.

With respect to the downstream signal, an OLT downstream TDM interface104 receives the TDM signal from the TDM PHY 74. An OLT downstream TDMGEM terminator 103 buffers the TDM signal to generate GEM. An OLTdownstream Ethernet interface 106 receives the Ethernet frame from theEthernet PHY 73. An OLT downstream Ethernet GEM terminator 105 generatesGEM. An OLT transmission scheduler 102 controls the OLT downstream TDMGEM terminator 103, and periodically transmits the GEM of the TDM to atransmission GEM buffer 101. The OLT transmission scheduler 102 alsocontrols an OLT downstream Ethernet GEM terminator 105, and transmitsthe GEM of the Ethernet frame to the transmission GEM buffer 101 at anidle timing. The OLT transmission scheduler 102 controls thetransmission GEM buffer 101, and periodically transmits the GEM of theTDM signal as well as the GEM of the Ethernet frame, to a transmissionGEM assembly 100. The transmission GEM assembly 100 assembles the GEMfor the payload of the PON frame, and transmits the assembled GEM to aPON transmission unit 99. The PON transmission unit 99 generates aheader, and then transmits the PON frame.

The ranging is performed to measure the distance between the OLT 1 andthe ONU 2, in such a way that a ranging controller 98 transmits aranging signal from the PON transmission unit 99 at a timing permittedby the OLT transmission scheduler 102. When a response from the ONU 2 isreturned to the ranging controller 98 through the PON reception unit 90,the ranging is completed.

Incidentally, the MPU 75 provides control to the control blocks throughan MPU interface 107.

FIG. 5 is a block diagram of the PON transceiver block of the ONU. Thedownstream signal from the photoelectric conversion module 81 isreceived by a PON reception unit 127. The PON reception unit 127performs such processes as synchronization and GEM extraction. Areception GEM assembly 126 assembles the GEM that is divided into pluralshort frames for transmission. The assembled GEM is stored in areception GEM buffer 125, and is transmitted either to an ONU downstreamEthernet GEM terminator 121 or to an ONU downstream TDM GEM terminator123, according to the table information of an ONU reception table 124.The ONU downstream Ethernet GEM terminator 121 extracts an Ethernetframe from the GEM. The Ethernet frame is transmitted to the EthernetPHY 83 through an Ethernet interface 120. The ONU downstream TDM GEMterminator 123 extracts a TDM signal from the GEM, and transmits the TDMsignal to the TDM PHY 84 through an ONU downstream TDM interface 122 ata predetermined timing.

With respect to the upstream signal, an ONU upstream TDM interface 134receives the TDM signal. An ONU upstream TDM GEM terminator 133 buffersthe TDM signal to generate GEM. An ONU upstream Ethernet interface 136receives the Ethernet frame. An ONU upstream Ethernet GEM terminator 135generates GEM. An ONU transmission scheduler 131 controls the ONUupstream TDM GEM terminator 133, and periodically transmits the GEM ofthe TDM to a transmission GEM buffer 132. The ONU transmission scheduler131 also controls the ONU upstream Ethernet GEM terminator 135, andtransmits the GEM of the Ethernet to the transmission GEM buffer 132 atan idle timing. The ONU transmission scheduler 131 controls thetransmission GEM buffer 132, and periodically transmits the GEM of theTDM as well as the GEM of the Ethernet, to a transmission GEM assembly130. The transmission GEM assembly 130 assembles the GEM for the payloadof the PON frame, and transfers the assembled GEM to a PON transmissionunit 129. The PON transmission unit 129 generates a header, and thentransmits the PON frame.

In the case of a ranging request, a ranging controller 128 processes aranging signal received by the PON reception unit 127, and returns aranging reception signal through the PON transmission unit 129.

Incidentally, the MPU 85 provides control to the control blocks throughan MPU interface 137.

Returning to FIG. 1, the OLT 1 measures the distance to each of the ONUs2-1 to 2-2, according to the ranging procedure indicated in ITU-TRecommendation G.984.3. The OLT 1 sets the equalization delay to eachONU 2 so that all the ONUs 2 appear to be located at an equal distancefrom the OLT 1. Because of this setting, it is possible to treat all theONUs 2 as if they are connected, for example, at 20 km. In addition,this prevents the upstream signals from the ONUs 2 from colliding witheach other on the trunk fiber 8.

FIGS. 6A and 6B are block diagrams of the PON reception unit and the PONtransmission unit in the OLT. FIGS. 7A and 7B are block diagrams of thePON reception unit and the PON transmission unit in the ONU. In FIG. 6A,the OLT PON reception unit 90 includes: a descrambler 901 for releasingthe scramble of the upstream signal; a frame synchronization unit 902for performing frame synchronization of the descrambled signal; an FECdecoder 903 for separating the information word and the FEC parity tocorrect an error of the information word; a PON frame terminator 904;and a decoder 905 for decoding the code. In FIG. 6B, the OLT PONtransmission unit 99 includes: an encoder 991 for encoding thedownstream signal; a PON frame generator 992 for making the encoded datainto a PON frame; an FEC encoder 993 for adding the FEC parity to thePON frame; a frame synchronization signal insertion unit 994 forinserting the frame synchronization signal; and a scrambler 995.

The ONU PON reception unit 127 of FIG. 7A has the same configuration asthe OLT PON reception unit 90 of FIG. 6A, excepting the signal flow.Similarly, the ONU PON transmission unit 129 of FIG. 7B has the sameconfiguration as the OLT PON transmission unit 99 of FIG. 6B, exceptingthe signal flow. Thus, their description will be omitted herein.

Referring to FIG. 8, a description will be given of another blockdiagram of the OLT, which described using FIGS. 2 and 4. Here, FIG. 8 isa functional block diagram of the OLT. In FIG. 8, a data interface 201receives the signals from the IP network 6 and the TDM network 7. Thesignals are once stored in a downstream data buffer 202. Based on thePON downstream frame signal format described in ITU-T RecommendationG.984.3, a downstream PON frame generator 203 stores the signal from thedownstream data buffer 202 into a GEM frame payload, and the signal froman upstream timeslot controller 214 into a granted field of thedownstream PON frame. The downstream PON frame transmitted from thedownstream PON frame generator is converted to a serial signal by aparallel/serial converter 204. The serial signal is converted from theelectrical signal to an optical signal by E/O 205. The converted opticalsignal is transmitted to the trunk fiber 8 through a WDM filter 207.

The upstream signal from the trunk fiber 8 is wavelength-divisiondemultiplexed by the WDM filter 207. The wavelength-divisiondemultiplexed upstream signal is converted from the optical signal tothe electrical signal by O/E 208. The converted electrical signal isidentified with respect to the value 0 or 1 by an Automatic ThresholdControl (ATC: identification unit) 209 using an appropriate threshold. Aclock extraction unit 210 performs clock extraction and retiming, fromthe identified signal. Further, a serial/parallel converter 211 detectsthe delimiter field of the PON upstream frame signal format that isdescribed in ITU-T Recommendation G.984.3, identifies the segments ofthe upstream signal, and converts the serial signal to parallel signals.An upstream PON frame terminator 212 identifies a user signal and acontrol signal included in the upstream frame, and outputs the usersignal to an upstream data buffer 213.

An upstream timeslot controller 214 extracts notification information(queue information), which is one of the control signals and indicatesthe transmission data accumulation states of the ONUs, from the upstreamPON frame terminator 212. The upstream timeslot controller 214calculates the upstream timeslot to be assigned to each ONU, based onthe bandwidth control information specified in advance by anadministrator as well as on the notified queue information. The upstreamtimeslot controller 214 periodically updates the content of an upstreambandwidth management table 215. Further, based on the managed upstreamtimeslot information, the upstream timeslot controller 214 determinesthe boundary of the upstream burst signal from each ONU, and notifiesthe ATC 209 of a reset signal 216. Still further, based on the managedupstream timeslot information, the upstream timeslot controller 214determines the ranging window field, and notifies the ACT 209 of anoffset switching signal 217 for discriminating the ranging window fieldand the other timeslot field.

The user signal output from the upstream PON frame terminator 212 isonce stored in an upstream data buffer 213, and is transmitted to the IPnetwork 6 or the TDM network 7 through the data interface 201.

The configuration of the OLT optical signal reception part will bedescribed with reference to FIG. 9. Here, FIG. 9 is a hardware blockdiagram of the O/E and the ATC that constitute the OLT optical signalreception part. In FIG. 9, O/E 208 includes an Avalanche Photo Diode(APD) 220 connected to a high-voltage bias source 221, and aTrans-Impedance Amplifier (TIA) 222.

The APD 220 is reverse biased by a high voltage, in which the receivedoptical signal is amplified by an avalanche effect and is converted toan electrical current. Because of this amplification operation, it ispossible to correctly identify the data when a high-speed optical signalof over 1 Gbits/s is input as a weak signal of about −30 dBm. Theconverted electrical current is converted to voltage by the TIA 222 thatincludes a resistance 223 and an amplifier 224.

In the ATC 209, the received signal voltage is identified by a thresholdset to a value obtained by adding the offset 218 to a value 229 which ishalf the amplitude. As a result, a signal identified as “0” or “1” isoutput. The output of an amplifier 225 is input to a transistor 227-1 inwhich the peak value is detected using a diode function thereof from abase to an emitter. Then, the output is stored in a capacitor 228 and isgiven as a value 229. The reset signal 216 is given to a transistor227-2 just before the reception of the signal from each ONU. Then, thevalue 229 stored in the capacitor 228 is discharged and is reset to “0”level.

The configuration of the ATC will be described further in detail withreference to FIGS. 10 and 11. Here, FIGS. 10 and 11 are block diagramsof the ATC and peripheral circuits.

In FIG. 10, ATC 209A includes the ATC 209 and a selector 232. The O/E208 inputs the voltage-converted received signal into the ATC 209. Theupstream timeslot controller 214 inputs an offset switching signal 217into the ATC 209A, in addition to the reset signal 216 for the rangingwindow. In response to the offset switching signal 217, the selector 232selects a ranging offset 230. At this time, the offset 218 input to theATC 209 has the same value as the ranging offset 230. With respect tothe timeslot other than the ranging window, the upstream timeslotcontroller 214 inputs the offset switching signal 217 into the ATC 209A,in addition to the reset signal 216 for the timeslot other than theranging window. In response to the offset switching signal 217, theselector 232 selects a bust data offset 231. The offset 218 input to theATC 209 has the same value as the bust data offset 231. The ATC 209Aidentifies the received data by the selected offset, and transmits tothe clock extraction unit 210.

In FIG. 11, ATC 209B includes two ATCs 209 and a selector 233. The ATC209-1 has an input fixed to the ranging offset 230. The ATC 209-2 has aninput fixed to the burst data offset 231. The O/E 208 inputs thevoltage-converted received signal into the two ATCs 209. In response tothe offset switching signal from the upstream timeslot controller 214,the selector 233 selects the output signal of the ATC 290-1 for thetimeslot of the ringing window, while selecting the output signal of theATC 209-2 for the timeslot other than the ranging window. The selectedoutput signal is transmitted to the clock extraction unit 210.

Referring to FIGS. 12 and 13, a description will be given of thereception of upstream burst signals and ranging transmissions. Here,FIG. 12 is a diagram illustrating the reception of upstream burstsignals and ranging transmissions with a constant offset. FIG. 13 is adiagram illustrating the reception of upstream burst signals and rangingtransmissions with different offsets.

In FIG. 12, the horizontal axis represents the time axis. The verticalaxis corresponding to the OLT 1 represents the reception level and thethreshold. The vertical axis from the reception level original point ofthe OLT 1 to the ONUs 2-1 and 2-2 represents the distance. Morespecifically, the distance between the OLT 1 and the ONU 2-1 is 10 km,and the distance between the OLT 1 and the ONU 2-2 is 20 km. The offsetfor the burst data and the offset for the ranging transmission withinthe ranging window are the same value. The threshold for the burst dataand for the ranging transmission is determined to be “1” or “0” based ona value higher than the half of the burst data amplitude by the amountof the offset.

FIG. 12 shows a state in which the ONUs 2-1, 2-2 are newly added to theoptical access network 10. More specifically, when the OLT 1 transmits aranging request 1, the ONUs 2-1 and 2-2 transmit ranging transmissionsto the OLT 1, respectively. At this time, a ranging window 1 is open,and the OLT 1 processes the first received ranging transmission of theONU 2-1. Next, when the OLT 1 transmits a ranging request 2, the ONU 2-2transmits a ranging transmission to the OLT 1. At this time, a rangingwindow 2 is open, and the OLT 1 processes the first received rangingtransmission of the ONU 2-2. Incidentally, the downward arrows in thefigure represent the reset signals that the upstream timeslot controller214 transmits to the ATC 218 by determining the boundary of the burstsignal as well as the end of the ranging window.

In FIG. 13, the offset for the burst data is the bust data offset value,and the offset for the ranging transmission is the ranging offset value.The ranging offset value is set to the same value as the offset of FIG.12, and the burst data offset value is set to be lower than the offsetof FIG. 12. As a result, the threshold for the burst data is closer tothe half of the amplitude, than the threshold of FIG. 12.

As described above, the burst data offset value can be made smaller thanthe ranging offset value, because the OLT 1 can predict from which ONU 2the burst data is received, according to the timing in the signalwaiting window (burst data waiting window). Further, even if thedelimiter detection is failed, the data can be discarded by the laterstage logic. On the other hand, in the ranging window used for theranging to measure distances, the OLT 1 cannot predict the timing atwhich the ranging transmission is received. When the offset is reducedin the ranging window, the white noise may be misidentified as thenormal ranging transmission. In order to prevent such amisidentification, it is necessary to maintain the offset value equal tohalf the difference between the reception level from the farthest ONU,and the white noise. The ranging can be performed repeatedly.

Referring to FIGS. 14A, 14B and FIGS. 15A, 15B, a description will begiven of the peak value, bottom value, threshold, and offset of theburst reception signal. Here, FIGS. 14A, 14B are diagrams illustratingthe peak value, bottom value, threshold, and offset of the burstreception signal with a high offset. FIGS. 15A and 15B are diagramsillustrating the peak value, bottom value, threshold, and offset of theburst reception signal with a low offset.

FIG. 14A shows the case in which the received optical power is large(the ONU-OLT distance is short). FIG. 14B shows the case in which thereceived optical power is small (the ONU-OLT distance is long). Here,the offset is the same offset value of FIG. 12, and the threshold isequal to half the sum of the peak value and the offset value. Asapparent from FIG. 14B, when the received optical power is small, thethreshold is close to the peak value, so that the probability ofmisidentifying “1” as “0” is high.

FIG. 15A shows the case in which the received optical power is large.FIG. 15B shows the case in which the received optical power is small.Here, the offset is the same offset value of FIG. 13, and the thresholdis equal to half the sum of the peak value and the offset value. In FIG.15B, the threshold is in the middle of the eye pattern even when thereceived optical power is small, so that the probability ofmisidentification is low.

The burst reception characteristics will be described with reference toFIG. 16. Here, FIG. 16 is a diagram showing the relationship between thereceived optical power and the error rate, with the amount of offsetgiven as the parameter. In FIG. 16, the vertical axis represents thesignal error rate, and the horizontal axis represents the receivedoptical power. When the received optical power is about −30 dBm, anerror rate of 1 e-12 (1×10⁻¹²) can be obtained independent of the amountof the offset. However, the error rate increases as the received opticalpower decreases. When the offset is high, the increase rate issignificant and the inclination of the PON bust receptioncharacteristics is steep. The application of FEC can correct an errorrate of 1 e-4 to the equivalent of 1 e-12. Coding gain is the differencebetween the received optical power providing the error rate 1 e-4 thatcan be corrected, and the received optical power providing the errorrate 1 e-12 without error correction. The coding gain is 2.5 dBm whenthe offset for the burst data is the same as for the ranging window,whereas the coding gain is achieved to be 5 dBm when the offset for theburst data is reduced.

As described above, according to the embodiment, it is possible tomoderate the inclination of the PON burst reception characteristics andto improve the FEC effects. As a result, it is possible to provide a PONsystem and an optical line terminal that have excellent characteristics.

1. A method for determining thresholds which identify received signalsinto the value 0 or 1, said method comprising the steps of: a first stepfor selecting offsets; a second step for calculating thresholds foridentification based on the offsets; a third step for identifying saidsignals using the thresholds; and a fourth step for repeating said firststep to said third step for all received signals.
 2. The method fordetermining thresholds according to claim 1, wherein: said offsetsinclude a first offset and a second offset, and the first step furthercomprises the steps of selecting said first offset during apredetermined time period and selecting said second offset during otherthan the predetermined time period.
 3. The method for determiningthresholds according to claim 2, said method further comprising step of:a fifth step for determining whether the predetermined time period hasexpired, previous to the first step.
 4. The method for determiningthresholds according to claim 3, wherein: said thresholds are equal tohalf the sum of the peak value and the first or the second offset, saidpredetermined time period corresponds to a period of receiving a rangingtransmission from optical network units to an optical line terminal,said optical network units and said optical line terminal constitute apassive optical network, optical signals which are received by theoptical line terminal during the period of receiving the rangingtransmission are ranging signals, and the determination of whether thepredetermined time period has expired is made based on upstream timeslotinformation which is allocated to the optical network units by theoptical line terminal.
 5. A method for determining thresholds whichidentify received signals into the value 0 or 1, said method comprisingthe steps of: a first step for calculating identification thresholdsbased on a first offset and a second offset; a second step foridentifying the signals using the calculated identification thresholds;a third step for selecting the identified signals based on a firstthreshold which is calculated based on the first offset during apredetermined time period and for selecting the identified receivedsignals based on a second threshold which is calculated based on thesecond offset during other than the predetermined time period.
 6. Themethod for determining thresholds according to claim 5, said methodfurther comprising step of: a fourth step for determining whether thepredetermined time period has expired, previous to the third step. 7.The method for determining thresholds according to claim 6, wherein:said thresholds are equal to half the sum of the peak value and thefirst or the second offset, said predetermined time period correspondsto a period of receiving a ranging transmission from optical networkunits to an optical line terminal, said optical network units and saidoptical line terminal constitute a passive optical network, opticalsignals which are received by the optical line terminal during theperiod of receiving the ranging transmission are ranging signals, andthe determination of whether the predetermined time period has expiredis determined based on upstream timeslot information which is allocatedto the optical network units by the optical line terminal.